This invention relates to a linear actuator and, in particular, to a bonding method for a laminated piezoelectric element which is used as a driving source of the linear actuator.
Previously, linear actuators using piezoelectric elements are used as auto-focus actuators or zoom actuators for use in cameras.
By way of illustration, Japanese Patent No. 2633066 (JP-B 2633066) (which will be also called a first patent document), which corresponds to U.S. Pat. No. 5,225,941, discloses a driving device comprising a driving rod frictionally engaged with a lens barrel, a piezoelectric element disposed in contact with the driving rod, and a leaf spring for bringing the driving rod into frictional engagement with the lens barrel. That is, the driving rod is bonded to an end of the piezoelectric element in an expansion direction. The lens barrel is movably supported to the driving rod. The leaf spring produces friction between the driving rod and the lens barrel. In the driving device disclosed in JP-B 2633066, a voltage is applied to the piezoelectric element so as to make a speed of expansion of the piezoelectric element different from a speed of contraction thereof.
In addition, Japanese Patent No. 3218851 (JP-B 3218851) (which will be also called a second patent document), which corresponds to U.S. Pat. No. 5,589,723, discloses a driving apparatus comprising a piezoelectric element, a driving shaft, coupled to the piezoelectric element, for extending in an expansion direction of the piezoelectric element, and a driven member (a lens barrel) frictionally coupled to the driving shaft. The driving apparatus in JP-B 3218851 drives the lens barrel by devising a driving signal applied to the piezoelectric element.
Furthermore, Japanese Unexamined Patent Application Publication NO. 2006-184565 (JP-A 2006-184565) (which will be also called a third patent document) discloses an optical module which is miniaturized and which can improve the positional accuracy of a lens holder. The optical module comprises the lens holder for holding lenses, a lens holder supporting body, a plurality of piezoelectric elements disposed rotationally symmetric about an optical axis of the lenses, and a plurality of weights which are coupled to the piezoelectric elements and which are disposed rotationally symmetric about the optical axis of the lenses. The lens holder is movable in the optical direction of the lenses. The lens holder supporting body comprises a cylinder portion therein and slidably supports the lens holder in an inner surface of the cylinder portion in the optical direction of the lenses. In addition, the lens holder supporting body holds the lens holder at an arbitrary potion in the optical direction of the lenses by means of static frictional force generated between the inner surface of the cylinder portion and an outer surface of the lens holder. A voltage is applied to the plurality of piezoelectric elements so as to make the speed of expansion thereof different from the speed of contraction thereof, thereby the piezoelectric elements expand and contract in the optical direction of the lenses. In addition, the plurality of piezoelectric elements have one surfaces in the expansion direction that are fixed to a surface of the lens holder at one end portion in the optical axis of the lenses. The plurality of weights are fixed to other surfaces of the piezoelectric elements in an expansion direction thereof.
At any rate, the optical module disclosed in JP-A 2006-184565 moves the lens holder by means of an “inertial force” of the weights generated by expansion and contraction of the piezoelectric elements. As the piezoelectric element, a laminated piezoelectric element where a plurality of piezoelectric layers (internal electrodes) are laminated is used. In JP-A2006-184565, the piezoelectric element has one end fixed to the lens holder (a movable portion) and another end fixed to the weights. That is, a combination of the piezoelectric elements, the lens holder, and the weights composes a lens unit. The lens unit is slidably supported by the lens holder supporting body in the optical direction of the lenses. In addition, in JP-A 2006-184565, each weight has another end which is fixed to no body and therefore the piezoelectric elements and the weights are put into a hung state where they are hung down over the lens holder (the movable portion).
In the above-mentioned first through third patent documents, in the manner which will be described in conjunction with FIG. 1, the piezoelectric element has upper and lower surfaces in the expansion direction that are uniformly bonded to a housing (a supporting member, a supporting plate, a weight fixing member) or a driving object (the driving rod, the driving shaft, the lens holder) via an adhesive agent. Herein, the housing and the driving subject will be collectively called bonded objects.
However, a related bonding method is disadvantageous in that a piezoelectric efficiency is decreased and it is in danger of stripping the adhesive agent.